The present invention relates to a honeycomb structure provided with a plug-in hole for a sensor.
To prevent environmental pollution and improve the environment, a catalyst converter is used for the treatment of an exhaust gas from automobiles. This catalyst converter converts harmful substances (nitrogen oxide, carbon monoxide, hydrocarbon, etc.) contained in the exhaust gas into components and/or the amount dischargeable to the environment in accordance with law regulations. When the exhaust gas passes through the catalyst converter, the harmful substances contained in the gas are decreased.
However, to confirm whether the harmful substances have actually been decreased or not, it is difficult to directly measure the concentration of the harmful substances in the exhaust gas by use of a sensor.
To solve the problem, the function of the catalyst converter is monitored instead. If the catalyst converter functions, the harmful substances must be decreased. When the function of the catalyst converter is monitored, there is adopted a means where oxygen sensors are disposed before and after the catalyst converter, respectively, and the content of oxygen in the exhaust gas is measured by use of these sensors to estimate the accumulation capacity of the catalyst and the proceeding of an aging process. Alternatively, there is adopted a means where heat sensors are disposed before and after the catalyst converter, respectively, and the temperature changes of the exhaust gas are measured by use of these sensors to predict whether the catalyst converter works or not (e.g., Japanese Patent Publication No. 2004-526564).
When the above sensor cannot be installed on the upstream side of the catalyst converter owing to space restriction or the like, heretofore a constitution has been adopted in which a hole is formed in a honeycomb structure of the catalyst converter to insert the sensor into the hole. The honeycomb structure including the sensor is installed in an exhaust system of the automobile (hereinafter appropriately referred to as “the conventional honeycomb structure”). Moreover, such a conventional honeycomb structure intends to cope with the space restriction.
However, this conventional honeycomb structure cannot sample any gas from portions other than the hole for the sensor. Furthermore, an only part of the exhaust gas selectively flows into the sensor (the measurement point of the sensor). Therefore, an engine cannot be controlled with an excess air ratio (λ)=1, and precise exhaust gas control cannot be performed. Moreover, a further problem occurs that it is difficult to precisely control the exhaust gas. Above all, this tendency becomes remarkable as the fluctuations of the excess air ratio (λ) between cylinders are large or as the diameter (the sectional area) of the honeycomb structure is large.
To solve these various problems, JP-A-2003-225576 is suggested as follows.
In JP-A-2003-225576, the main surface of partition walls of a honeycomb-like fired article is cut in an oblique cut proceeding direction to provide at least one of a groove, a hole and an edge in the honeycomb-like fired article, whereby the sensor space is secured and the decrease of the catalyst capacity is minimized to obtain a desired purification performance. In this honeycomb of JP-A-2003-225576, when the sensor space having a desired shape is provided, an attempt to adequately control the inflow of the exhaust gas into the sensor is eventually recognized. In this respect, this constitution is appreciated to a certain degree. However, since any gas cannot be sampled from the portions other than the hole for the sensor and the exhaust gas flowing into the sensor stays in a part, the engine cannot be controlled with the excess air ratio (λ)=1, and it is difficult to precisely control the exhaust gas in a case where the excess air ratio (λ) noticeably fluctuates between the cylinders. Therefore, it cannot be considered yet that the constitution is sufficient.
Thus, at present, there is not any sufficient countermeasure for the above-mentioned problems, and a further improvement is demanded.